The present invention relates generally to improved methods for evaluating subsurface fracture parameters in conjunction with the hydraulic fracturing of subsurface formations, and more specifically relates to improved methods for utilizing test fracture operations and analysis, commonly known as "mini-frac" operations, to design formation fracturing programs.
Mini-frac operations consist of performing small scale fracturing operations utilizing a small quantity of fluid to create a test fracture and to determine pressure decline data of the formation. Mini-frac operations are performed using little or no proppant in the fracturing fluid. After the formation is fractured, the well is shut in and the pressure decline of the formation is observed over time. The data thus obtained is used in a fracture model to determine parameters to be used to design the full scale formation fracturing treatment.
Mini-frac test operations are significantly different from conventional full scale fracturing operations. For example, as discussed above, only a small amount of fracturing fluid is injected (for example, as little as 25 barrels), and no proppant is typically utilized. The desired result is not a propped formation fracture of practical value, but a small scale, short duration, fracture, to facilitate collection of pressure decline data in the formation. This pressure decline data will facilitate estimation of formation and fracture parameters.
For example, the pressure decline data will be utilized to calculate the effective fluid loss coefficient, the fracture width and fracture length, the fracture fluid efficiency and the observed closure time. These parameters will then be utilized in a fracture design system to design the full scale fracturing operation. Accurate knowledge of the fluid leak-off coefficient is of major importance in designing a fracturing operation. If the leak-off coefficient is estimated too low, there is a substantial likelihood of a sand-out. Conversely, if the fluid leak-off coefficient is estimated too high, too great a fluid pad volume will be utilized, thus resulting in significantly increased costs to the fracturing operation. Additionally in this circumstance, the use of fluid loss additives in the fracturing fluid to help counteract the effects of a estimated high leak-off coefficient will not only be costly, but may often cause damage to the formations.
Conventional methods of mini-frac analysis have required reliance upon assumptions of questionable validity. Conventional mini-frac analysis techniques have assumed that the width of a mini-frac test fracture is proportional to the pressure drop from the instantaneous shut-in pressure to the formation closure pressure. However, the mechanical properties of the fracturing fluid will have substantial impact upon the fracture dimensions. For example, a "thin", or relatively non-viscous, fracturing fluid will yield a long, narrow fracture; while a "thick", or relatively viscous, fracturing fluid, under the same conditions, will yield a fracture of significantly decreased length and increased width.
The mechanical properties of fracturing fluids can be expressed in known terminology in terms of a "fluid behavior index", and a "fluid consistency index". Conventional techniques of mini-frac analysis have failed to consider the rheology of the fluid, and have thus been unsuited to yielding optimal data regarding the mini-frac test fracture, leading to less than optimal data of the formation characteristics.
Accordingly, the present invention produces a new method for mini-frac analysis of formations and for designing subsurface of fracturing operations in response to the fracturing fluid rheology.